In the past, in the industrial production process comprising an oxidation step, in order to carry out a next step after terminating the oxidation step, an end point of the oxidation reaction is assessed while confirming the amount of an unreacted residue of the starting material with the use of a thin layer chromatography or the like, and the next treatment is carried out. For example, as the method for producing a 2-(2-pyridylmethylsulfinyl)benzimidazole compound among various products produced by oxidation, there has known a method of oxidizing a corresponding 2-(2-pyridylmethylthio)benzimidazole compound with meta chloro perbenzoic acid (e.g., see U.S. Pat. No. 4,255,431 and European Unexamined Patent Publication No. 80,602).
As the method for obtaining sulfoxide from sulfide in general, there has been known a method of oxidizing with the use of peroxy acid, hydrogen peroxide, iodosobenzene, N-halosuccinimide, tertiary butyl hypochloride, sodium metaperiodate, selenium dioxide, bromine, chlorine, or ozone (e.g., see The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and Their Sulphur Analogues, Supplement E, Part 1, pp. 539-608, Saul Patai, Ed., John Wiley and Sons, An Interscience Publication, (1980); and Synthesis of Sulfoxides by Oxidation of Thioethers, Michel Madesclaire Ed., Tetrahedron Report Number 210, Tetrahedron, 42, 5459-5495 (1986)), and the like.
In addition, there has been described a method of producing a 2-2(pyridylmethylsulfinyl)benzimidazole compound in the presence of a vanadium compound with the use of hydrogen peroxide as an oxidizing agent in Japanese Unexamined Patent Publication No. 1-131176.
However, it is not easy to determine the end point of the oxidation reaction in the reaction of oxidizing a 2-(2-pyridylmethylthio)benzimidazole compound to produce a 2-(2-pyridylmethylsulfinyl)benzimidazole compound with the use of any oxidizing agent and as well as in the oxidation reactions of obtaining other oxidized compounds, and thus in an industrial production process, the end point of the oxidation reaction is determined to move on to the next step by confirming the amount of an unreacted residue of the starting material with the use of a thin layer chromatography (hereinafter, also may be referred to as TLC). Therefore, it is not a method advantageous for an industrial production, and the production method which more simply determines the end point of the oxidation reaction and swiftly moves on to the next step has been demanded.
The present inventors have conducted extensive studies to discover a production method advantageous in an industrial practice, with which 2-(2-pyridyl- or phenyl-methylsulfinyl)imidazoles can be produced from 2-(2-pyridyl- or phenyl-methylthio)imidazoles in good yield, less amount of byproducts such as corresponding sulfone products and 2-(2-pyridyl- or phenyl-methylsulfonyl)imidazole-N-oxides produced by overreaction are produced, and a computational management or the like is possible. Accordingly, they found that the electric potential for the oxidation of the 2-(2-pyridyl- or phenyl-methylthio)imidazoles to 2-(2-pyridyl- or phenyl-methylsulfinyl)imidazoles increases as the reaction proceeds and rapidly decreases as the reaction comes to the end, and as a result of the further investigation, they have completed the invention.